The present invention relates generally to optical amplifiers, and more particularly to a Raman amplifier having a relatively wide gain profile that is also substantially flat over the bandwidth of the optical signal being amplified therein.
Broad bandwidth optical transmission systems have received a great deal of attention in recent years. Such systems require broad bandwidth optical amplifiers to achieve transmission of high capacity wavelength division multiplexed signals. A type of optical amplifier that is sometimes employed is a so-called distributed amplifier in which signal amplification occurs along the signal transmission path. An example of a distributed amplifier is a Raman amplifier.
Raman amplification is accomplished by introducing the signal and pump energies along the same optical fiber. The pump and signal may be copropagating or counterpropagating with respect to one another. A Raman amplifier uses stimulated Raman scattering, which occurs in silica fibers when an intense pump beam propagates through it. Stimulated Raman scattering is an inelastic scattering process in which an incident pump photon loses its energy to create another photon of reduced energy at a lower frequency. The remaining energy is absorbed by the fiber medium in the form of molecular vibrations (i.e., optical phonons). That is, pump energy of a given wavelength amplifies a signal at a longer wavelength. The relationship between the pump energy and the Raman gain for a silica fiber is shown in FIG. 1. The particular wavelength of the pump energy that is used in this example is denoted by reference numeral 1. As shown, the gain spectrum 2 for this particular pump wavelength is shifted in wavelength with respect to the pump wavelength. As FIG. 1 indicates, the bandwidth of the Raman amplifier is limited. For example, the bandwidth of the amplifier shown in FIG. 1 is only about 20 nm at a gain of 10 dB.
U.S. Appl. Ser. No. [Kidorf 7-5] discloses a Raman amplifier that has an increased bandwidth. This result is accomplished by providing two pump sources providing pump energy at two or more different wavelengths. As shown in FIG. 2, pump energy supplied at a wavelength denoted by reference numeral 40 generates gain curve 42 while pump energy supplied at a wavelength denoted by reference numeral 41 generates gain curve 43. The composite gain spectrum, indicated by curve 44, has a bandwidth that is greater than either of the individual gain curves 42 and 43.
It is generally desirable to provide a gain spectrum that is reasonably flat over all or a substantial portion of the amplifier""s bandwidth. Unfortunately, if multiple pumps are used in the manner described in the previously mentioned patent, a flat gain spectrum will not necessarily arise. For example, if multiple pumps are employed that all generate the same pump power but at different wavelengths that are evenly spaced from one another, an asymmetric gain curve is produced in which more gain is generated at higher wavelengths than lower wavelengths. This occurs because the pump sources operating at lower wavelengths effectively pump the pump sources operating at higher wavelengths.
Accordingly it would be desirable to provide a wide-bandwidth Raman amplifier employing multiple pump sources in which the resulting gain spectrum is flat to within some specified tolerance over the amplifier""s bandwidth.
In accordance with the present invention, a Raman amplifier is provided that includes at least a portion of optical fiber in which an optical signal travels. The optical fiber portion may encompass all or part of the optical transmission path of an optical communication system. A pump energy unit is provided that includes at least three pump sources providing pump power at different pump wavelengths. The different pump wavelengths are spaced apart from one another by nonuniform amounts so that a prescribed Raman gain profile is generated in the optical fiber portion. An optical coupler is provided for coupling the pump power to the optical fiber portion and transmitting the optical signal therethrough.
In accordance with another aspect of the invention, the gain profile of the amplifier is substantially flat within a predetermined threshold over the bandwidth of the optical signal. Such a profile may be achieved by arranging the nonuniform pump wavelength distribution so that the spacing between pump wavelengths decreases for decreasing pump wavelengths. Moreover, the coupler coupling the pump power to the optical fiber portion may be a wavelength division multiplexed coupler, or, alternatively, an optical circulator. The pump energy unit may also include an optical coupler for combining the different pump wavelengths prior to being directed to the optical fiber portion in which the signal travels.
In accordance with yet another aspect of the invention, a method is provided for generating a prescribed Raman gain profile in at least a portion of optical fiber that serves as a gain medium and in which an optical signal travels. Pump power is provided at least three different pump wavelengths. The different pump wavelengths are selected so that they are spaced apart from one another by nonuniform amounts so that Raman gain generated in the optical fiber portion has a prescribed profile. The pump power is coupled to the optical fiber portion.